The present invention relates to a fixing device usable with an image forming apparatus such as a copying machine or a printer using an electrophotographic process, an electrostatic recording process or the like. In conventional image forming apparatuses, a toner image is formed in an image forming station through an indirect (transfer) type or direct type electrophotographic process, electrostatic recording process, magnetic recording process or another known process; and the formed toner image is transferred onto a recording material such as a transfer sheet, recording material, photosensitive paper electrostatic recording paper or the like; and the toner image is fixed on the recording material by a heat fixing device of a heating roller type into a permanent image.
The fixing device or the heating roller type comprises a fixing roller containing therein a heater (heating roller) and a pressing roller press-contacted thereto for form a nip, into which the recording material is introduced and through which the recording material is passed, by which the toner image on the recording material is fixed on the surface of the recording material by heat and pressure.
Recently, the demands with respect to the heat fixing device is (1) high speed start-up and (2) unnecessity for stand-by temperature control, in terms of the convenience and reduction of energy consumption.
In order to accomplish the high speed start-up, it would be considered to reduce the time required for temperature rise up to a desired temperature by reducing the thermal capacity by reducing the thickness of the heat roller, and by using an IH (electromagnetic induction heating type) as a heat source.
A temperature control system in the IH system, it would be considered to use ON-OFF control system which has been used for a temperature control for a halogen lamp (heat source). However, use of the ON-OFF control system with the heat roller having a small thermal capacity results in abrupt temperature changes occurring, and therefore, the proper control is difficult.
Then, a feed-back type is being considered wherein electric power supplied to the heat source used in the IH type system is linearly controlled in accordance with a result of temperature measurement of the fixing device (FIG. 8). In FIG. 8, the FIG. 8 represents an electric power value Po supplied to the heat source, and the abscissa represents a temperature T. When the temperature of the fixing device is Ta, the electric power Pa is supplied to the heat source.
Generally, the relation between the electric power supplied to an induction coil and the electric power efficiency is as shown in FIG. 9 in the IH system. In FIG. 9, the ordinate represents an electric power efficiency xcex7, and the abscissa represents an electric power Po supplied to the heat source. When the electric power of 200W is supplied to the induction coil, the electric power efficiency is approx. 88%, but when the electric power not more than 200W is supplied, the electric power efficiency is significantly lower than that.
The fixing temperature in the image forming operation is generally not higher than 210xc2x0 C. When the electric power of 200W is supplied to the coil constituting the IH type voltage source, the temperature may exceed 210xc2x0 C.
However, if the range not higher than 200W is used, the electric power efficiency is not good, as described above, and therefore, doing so is not advantageous from the standpoint of energy conservation.
Accordingly, it is a principal object of the present invention to provide a fixing device of an electromagnetic induction heating type wherein the electric power efficiency is high.
It is another object of the present invention to provide a fixing device wherein an electric power value exhibiting a low electric power efficiency is not used.
According to an aspect of the invention, there is provided a fixing apparatus comprising an induction coil for generating a magnetic field upon electric power supply thereto; a fixing rotatable member for induction heat generation by the magnetic field generated by said induction coil; temperature detecting means for detecting a temperature of said fixing rotatable member; determining means for determining an electric power value to be supplied to said induction coil on the basis of a detected temperature by said temperature detecting means; and control means for controlling the electric power supply to said induction coil on the basis of the electric power value determined by said determining means, wherein said control means substantially continues to supply the electric power while the electric power value determined by said determining means is higher than a predetermined value, and effects an ON/OFF control of rendering on and off the electric power supply when the electric power value is lower than the predetermined value.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings.